Anti-foulant additives are commonly used in liquid processes to reduce or prevent the buildup of solids, often referred to as scale, on a surface of an apparatus. In processes in which steam is produced, such as steam applications and steam boilers, for example, an anti-foulant additive assists in the reduction or prevention of scale formation on the surfaces of the process apparatus.
Mills that process and produce cellulosic pulp are comprised of digesters, screens, recirculation heaters, and top separators that are subject to scale formation. Scale may form both by the direct precipitation of dissolved species and by the deposition of suspended solids contained within the process fluids and other liquids that pass through these devices. The buildup of scale in the pulp process usually results in lost revenue due to more frequent shut downs of paper-making machinery for cleaning, as well as increased steam demand (resulting in increased operating costs) to offset a loss in heat transfer efficiency.
Anti-foulant additives are frequently effective in reducing scale buildup, thereby maintaining good heat transfer rates and resulting in possibly fewer washing shut downs. However, mill trials for anti-foulants are costly, requiring several months due to infrequent shutdowns for washing when scale can be observed. In addition, results integrated over several months are difficult to interpret because of process fluctuations and variations in the time since last cleaning.
Several prior art processes and devices for testing the effectiveness of anti-foulants include a core component consisting of a heated surface enclosed within a conduit to form a heat transfer section for the purpose of monitoring scale. These processes involve variations on the physical configuration of the heated surface or in how the scale accumulation is monitored. Some methods employ the continuous flow of a sample fluid though a test section. Other methods require the test section to be immersed in a fluid bath and employ an impeller to recirculate the fluid through the test section. Yet other designs also employ removable surfaces for testing different alloys for comparison. In all cases, the methods are designed to conduct a single test. If the effect of changing conditions is to be evaluated, including temperature, fluid velocity, and anti-foulant concentration, then a series of tests must be conducted for each tested condition.
The problem with testing for scale formation and evaluating the effect of anti-foulants thereon is that the phenomenon of fouling is not fully understood from a fundamental point of view. Heat exchange fouling models are partly empirical. The skilled artisan can attest that significant variations may occur in fouling tests conducted in the same apparatus under the same conditions. Therefore, many repeated tests are required to get an average value of the fouling rate for any given set of conditions.